The proposed research is an integral part of an ongoing effort in our laboratory to discover and characterize the neural bases of associative learning. The preparation being explored is the classically conditioned rabbit nictitating membrane, and the research covered by this proposal is the neuroanatomical identification of neurons which may mediate the unconditioned response (UCR) and others which transmit sensory information to the sensory-motor cortex. For the UCR, sensory input to the central nervous system occurs through the ophthalmic branch of the trigeminal nerve while the motor output is mediated by the sixth (adbucens) cranial nerve. Specific neurons within these structures will be identified by the proposed research as will those mediating the response between the structures. Although future neurophysiological and behavioral research will be necessary to conclusively demonstrate neural connections as the neurons mediating the UCR, the present work will identify the anatomical candidates for this response as well as those for mediating sensory information to the cortex. Both anterograde and retrograde transport of horseradish peroxidase (HRP) will be used as tools for the neuroanatomical identification of pathways.